Pinion Feast
Instant
Destroy target creature with flying. Bolster 2. (Choose a creature with the least toughness among creatures you control and put two +1/+1 counters on it.)
"Even paradise is not without tragedies." —Sarkhan Vol
Illustrated by
Dragons of Tarkir#195CommonEnglishLegality
Rulings
You must choose a creature with flying in order to cast Pinion Feast. You can’t cast it without a legal target just to bolster.
Bolster itself doesn’t target any creature, though some spells and abilities that bolster may have other effects that target creatures. For example, you could put counters on a creature with protection from white with Aven Tactician’s bolster ability.
You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves. That could be the creature with the bolster ability, if it’s still under your control and has the least toughness.